Lithography apparatuses are used for example in the production of integrated circuits or ICs in order to image a mask pattern in a mask onto a substrate, such as e.g. a silicon wafer. In this case, the mask generates a circuit pattern corresponding to a respective layer of the IC. This pattern is imaged onto a target region of the silicon wafer that is coated with a photoresist. In general, a single silicon wafer includes a multiplicity of target regions adjoining one another, which are gradually exposed. In this respect, a distinction is drawn between two types of lithography apparatuses. In the case of the first type, the target region is exposed by the entire mask being exposed in one step. This type of apparatus is usually designated as “stepper”. A second type of lithography apparatus—usually designated as “step-and-scan” apparatus—provides for each target region to be illuminated by the mask being progressively scanned with a light beam. Synchronously therewith, the substrate is scanned by the light beam.
In order to enable very dense and small structures to be produced, so-called multiple patterning methods have been disclosed. These include double patterning or quadruple patterning, for example. In this case, difference sequences of exposure and etching steps are used in order to be able to produce particularly small patterns on substrates, which otherwise could no longer be imaged sufficiently sharply.
A further step towards sharper imaging during photolithographic patterning is so-called immersion lithography. In the latter, air in the gap between the last lens element and the wafer surface is replaced by an immersion liquid having the highest possible refractive index. This technique allows structures of minimally 28 nm to be manufactured in industrial mass production using existing lithography systems on the basis of ArF excimer lasers (also called 193-nanometer lithography).
Yet another step in the development of improved lithography apparatuses is so-called EUV lithography, which makes use of electromagnetic radiation having a wavelength of 13.5 nm (also referred to as extreme ultraviolet radiation).
What is common to the lithography apparatuses described above is that the optical systems and elements have to be positioned highly accurately, in particular in order to minimize or avoid image distortions, unsharpnesses and an overlap offset during the imaging of the mask pattern on the substrate. This aspect is accorded a prominent importance precisely in the field of the 10 nm-technology node.
In this case, the positioning can include the positioning of optical and non-optical elements in up to six degrees of freedom (that is to say, for example, translationally along the three orthogonal axes and rotationally in each case about the same). The positioning can be adversely influenced by vibrations and the like. Such vibrations can arise for example outside the lithography apparatus, such as, for example, as a result of sound, vibrations of the foundations or the like, or within the lithography apparatus, such as, for example, as a result of reaction forces upon the actuation of, in particular, optical elements. External disturbing forces on the lens element or the lens, which typically weighs more than one ton, can vary in the range of 0.1 N or even just 0.05 N, for example. The reaction forces that arise during a dynamic correction, in particular in real time, are much greater by comparison therewith. Despite the use of light materials and corresponding lightweight construction techniques, the forces used here are up to 10 N per degree of freedom, for example. Forces of 50 N, for example, can thus arise in total, such that force suppression by a factor of 1000 may be used.
By way of example, EP 1 321 823 A2 discloses in its FIG. 3 a mirror 10, which is held relative to a system frame 11 (also referred to as “lens barrel”) by gravitational force compensation springs 12. An actuation for altering the position of the mirror 10 is carried out via actuators 15, for example Lorentz force motors, which are supported on the system frame 11 via a reaction mass 14 and a spring 16. Upon an actuation of the mirror 10 via the actuator 15, the reaction mass 14 reduces the reaction forces of the actuator 15 that are transmitted to the system frame 11. The natural frequency of the reaction mass 14 and assigned spring 16 is in this case typically 10 Hz, and that of the mirror 10 and of assigned gravitational force compensation springs 12 is typically significantly less than 1 Hz.